In general, a conventional oscillator can be implemented by a variety of types of oscillators. For example, the conventional oscillator may include an LC resonator. Unfortunately, a frequency drift is often introduced due to the process variations or temperature changes. In order to calibrate the frequency drift, a conventional method is employed to adjust the capacitor array of the resonator (e.g. the LC resonator) of the oscillator, so as to obtain an accurate frequency. However, this may cause the design of the oscillator become more complex because the analog control approach is sensitive to PVT variation, especially in advanced process. Therefore, it is important to provide a novel scheme to simultaneously achieve high frequency accuracy and easy implementation by digital adjustments.